Regarding the concept of introducing hydrogen gas into natural gas pipelines, this is indeed a hot topic and there are recent quantitative treatments of fatigue crack growth driven by pressure cycling and potentially accelerated by hydrogen.  Some analysis has shown that it can be acceptable to operate natural gas pipelines with a hydrogen blend.  However, this is highly dependent upon the pressure and wall stress.  Whereas low pressure distribution lines with low wall stress are more amenable, the higher pressures and wall stresses of major transportation pipelines may not be.  Deep cyclic stresses that might occur from using pipelines as storage  may also create additional issues since hydrogen can accelerate fatigue crack growth, especially for systems where both hydrogen and deep cycles were not anticipated in the original design.  Individual pipelines likely need to be evaluated based on their design and there is likely to be no single answer for this question of pipeline storage. 

    
For industrial propane tanks, the Panel needs more information about structural materials and their properties (including welds). All propane tanks are not built from the same materials or with the same construction techniques, so these tanks likely need to be evaluated on a case-by-case basis. Propane tanks are often built with techniques that leave potential internal features that are susceptible to the initiation of crack growth. In addition, and depending on the application, it’s likely that the cyclic pressure service will be significantly different for a tank in hydrogen service, particularly one that might be cycled deeply on a daily (or more frequent) basis. From the perspective of the low-pressure storage options, while it is tempting to repurpose old LPG vessels for hydrogen service, the Panel cautions against it due to the potential for hydrogen embrittlement in the steel/weld material. Also, a challenge with newer propane tank designs is that they are moving to thinner walls and higher strength steel, which is notably less resistant to embrittlement than older vessels, so counterintuitively, newer tanks might be less amenable to hydrogen than older tanks.   

Another concern for vessels in hydrogen service is the amount of non-circularity, or “peaking” of the longitudinal welds.  This is not as much of a concern for propane so is likely not to be part of a standard design specification. This manufacturing issue can have a severe effect on cyclic life and is well understood when designing and building high cyclic hydrogen vessels, but not propane. Besides the issue described above for the tank itself, there are similar potential embrittlement issues for PRVs, instrumentation, and other accessories on these tanks. Many incidents have been due to component failures in these accessories. 
In conclusion, propane vessels were not designed for H2 cyclic service. Almost by definition, the reason people want to use them is for storage. and that is likely to be over a wide pressure range. Fracture mechanics should be applied to all cyclic H2 service vessels, especially if they start from another service. Propane tank designs vary and unless a tank is specifically intended for H2, there is no guarantee of what might be used terms of materials and construction techniques.